The present invention relates to a control system for a robot having a servo motor for driving a robot operation axis. The control system is capable of limiting the torque and also causing the robot operation axis to "retract" when the position error of the axis becomes excessive.
When an operating axis of a moving part, such as an arm, of a robot is driven by a servo motor, a position control system, such as that shown in FIG. 5, is generally adopted.
In FIG. 5 reference character B* denotes pulses whose frequency is proportional to a reference (or target) velocity. The pulses B* are supplied from a suitably programmed computer, not shown. The particular program used varies depending on the operation being performed by the robot. For instance, when a part is to be inserted into a hole of a workpiece, as shown in FIG. 6, the work schedule essentially consists of moving the holder of the part to a first point above the insertion hole, and lowering the vertical axis to a second point at which the part is fully inserted.
The reference character B denotes pulses whose frequency is proportional to the actual velocity of the servo motor 4 as detected by a pulse generator 5.
An adder 1a determines the difference between the reference velocity B* and the actual velocity B and produces pulses indicating the difference .DELTA.B of the velocity. A position error counter 1 counts the pulses and its count value indicates the position error of the operation axis driven by the servo motor 4 from the desired instantaneous position. A D/A (digital to analog) converter 2 converts the digital signal from the counter 1 into an analog signal serving as a speed reference V*, which is then applied to a drive unit 3 for driving a servo motor 4.
If, for some reason, movement of the operation axis and hence rotation of the servo motor 4 are is obstructed, the position error .DELTA.B increases and the position error counter 1 overflows.
In a prior art system, the overflow of the counter 1 is detected and the robot is made to stop or some other remedy against the error is made.
A disadvantage of the prior art system is that when the rotation of the motor 4 is obstructed an over current flows through the servo motor 4 to cause burning of the servo motor 4 and an excessive torque of the servo motor 4 may occur, which may damage the robot. Such an over current and an excessive torque continue until the counter 1 overflows and the motor current is terminated.
For instance, assume that a robot 20, as shown in FIG. 6 is used to hold a part 30 by a holder 61 at an end of an arm 60 of a vertical axis and to insert the part 30 into an insertion hole 40. If the arm 60 and the insertion hole 60 is out of alignment, insertion of the part 30 is obstructed, and for this reason an over current flows through the servo motor 24 for driving the vertical axis and also the part 30 and the insertion hole 40 may be damaged.
Incidentally, reference numerals 21 and 22 in FIG. 6 denote servo motors for positioning the vertical axis, reference numeral 23 denotes a servo motor for a part holding operation, and reference numeral 70 denotes a control unit.